There is not really a best subwoofer . You plan on running a 2000 watt rms amp?? How much space do you have? Are you going ported or sealed? I heard a Sundown NS v.2 15 in a ported enclosure he said about 3.5 cu ft tuned 32hz running off of a Sundown saz 2500 and it slammed.

I have also heard Kicker 15L7 in a sealed box using a kicker ix 1000,1 and it was impressive. The Kicker almost has the cone area of a round 18" sub.

There are a lot of options out there just depends on what you want to accomplish.

I'm never sure how to answer a "what's best" question. It's similar to what is the best horse? Well, for plowing the field or for running the quarter mile?
One woofer will not own the best of ALL qualities. So what are you looking for? SQ or SPL? Boat or car? Power handling or responsiveness? What is the target enclosure size if that matters?

Best at what?? Not in a specific order but all worth mentioning...
Kicker L7 you'll want atleast a 1500 watts... Solid Stillwater design and its own unique sound... Perfect for tight spaces and SQ sealed but can be a SPL monster ported right. ( don't use kicker specs ) have one of these in a car off a 2500 watt ZX amp..
MTX thunder9500 you'll want atleast 2000 watts.... Longtime player in the game and one of there top models... Ported it can move some air... I've only heard this, no hands on experience...
DC Audio, really depends on your "level"? 4 would be comparable I'd say to the above power and sound wise... Touchy speaker IMO.. But some just want it...
Sundowns sa15 is a solid 600 watt sub that can handle much much more and less expensive than some .. SPL woofer for sure.
Ascendant audio arsenal15 is also a good sub that can take alot of power beyond specs.. SPL woofer.
Audio ban immortal15 is a hard hitting SPL moster as well had 2 of these in a blazer once with 4000 watts to each..
JL 13w7 is a 13.5 but a strong contender here in the 15" catagory.. SQ is top notch if that's what you want... Can handle 2000 watts and purrs but it hits hard as well...

So far I think that's all the 15" speakers worth mentioning for brain shaking bass.... This is just my opinion and experience...